Apparatus and method for in situ heat processing of hydrocarbonaceous formations

ABSTRACT

The disclosure describes a technique for uniform heating of relatively large blocks of hydrocarbonaceous formations in situ using radio frequency (RF) electrical energy that is substantially confined to the volume to be heated and effects of dielectric heating of the formations. An important aspect of the disclosure relates to the fact that certain hydrocarbonaceous earth formations, for example raw unheated oil shale, exhibit dielectric absorption characteristics in the radio frequency range. In accordance with the system of the invention, a plurality of conductors are inserted in the formations and bound a particular volume of the formations. The phrase &#34;bounding a particular volume&#34; is intended to mean that the volume is enclosed on at least two sides thereof. Electrical excitation is provided for establishing alternating electric fields in the volume. The frequency of the excitation is selected as a function of the dimensions of the volume so as to establish a substantially non-radiating electric field which is substantially confined in the volume. In this manner, volumetric dielectric heating of the formations will occur to effect approximately uniform controlled heating of the volume.

BACKGROUND OF THE INVENTION

This invention relates to the exploitation of hydrocarbon-bearing earthformations, and, more particularly, to a system and method for the insitu heating processing of hydrocarbon-bearing earth formations such asoil shale, tar sands, coal, heavy oil, and other bituminous or viscouspetroliferous deposits. The present subject matter is related to subjectmatter set forth in the copending U.S. application Ser. No. 828,904, ofJack Bridges, Allen Taflove and Richard Snow, filed of even dateherewith and assigned to the same assignee as the present application.

Large scale commercial exploitation of certain hydrocarbon-bearingresources, available in huge deposits on the North American continent,has been impeded by a number of problems, especially cost of extractionand environmental impact. The United States has tremendous coatresources, but deep mining techniques are hazardous and leave a largepercentage of the deposits in the earth. Strip mining of coal involvesenvironmental damage or expensive reclamation. Oil shale is alsoplentiful in the United States, but the cost of useful fuel recovery hasbeen generally noncompetitive. The same is true for tar sands whichoccur in vast amounts in Western Canada. Also, heavy or viscous oil isleft untapped, due to the extra cost of extraction, when a conventionaloil well is produced.

Materials such as oil shale, tar sands, and coal are amenable to heatprocessing to produce gases and hydrocarboneous liquids. Generally, theheat develops the porosity, permeability and/or mobility necessary forrecovery. Oil shale is a sedimentary rock which, upon pyrolysis ordistillation, yields a condensable liquid, referred to as a shale oil,and non-condensable gaseous hydrocarbons. The condensable liquid may berefined into products which resemble petroleum products. Oil sand is anerratic mixture of sand, water and bitumen with the bitumen typicallypresent as a film around water-enveloped sand particles. using varioustypes of heat processing the bitumen can, with difficulty, be separated.Also, as is well known, coal gas and other useful products can beobtained from coal using heat processing.

In the destructive distillation of oil shale or other solid orsemi-solid hydrocarbonaceous materials, the solid material is heated toan appropriate temperature and the emitted products are recovered. Thisappears a simple enough goal but, in practice, the limited efficiency ofthe process has prevented achievement of large scale commercialapplication. Regarding oil shale, for example, there is no presentlyacceptable economical way to extract the hydrocarbon constituents. Thedesired organic constituent, known as kerogen, constitutes a relativelysmall percentage of the bulk shale material, so very large volumes ofshale need to be heated to elevated temperatures in order to yieldrelatively small amounts of useful end products. The handling of thelarge amounts of material is, in itself, a problem, as is the disposalof wastes. Also, substantial energy is needed to heat the shale, and theefficiency of the heating process and the need for relatively uniformand rapid heating have been limiting factors on success. In the case oftar sands, the volume of material to be handled, as compared to theamount of recovered product, is again relatively large, since bitumentypically constitutes only about ten percent of total, by weight.Material handling of tar sands is particularly difficult even under thebest of conditions, and the problems of waste disposal are, of course,present here too.

There have been a number of prior proposals set forth for the extractionof useful fuels from oil shales and tar sands in situ but, for variousreasons, none has gained commercial acceptance. One category of suchtechniques utilizes partial combustion of the hydrocarbonaceousdeposits, but these techniques have generally suffered one or more ofthe following disadvantages: lack of precise control of the combustion,environmental pollution resulting from disposing of combustion products,and general inefficiency resulting from undesired combustion of theresource.

Another category of proposed in situ extraction techniques would utilizeelectrical energy for the heating of the formations. For example, in theU.S. Pat. No. 2,634,961 there is described a technique whereinelectrical heating elements are imbedded in pipes and the pipes are theninserted in an array of boreholes in oil shale. The pipes are heated toa relatively high temperature and eventually the heat conduits throughthe oil shale to achieve a pyrolysis thereof. Since oil shale is not agood conductor of heat, this technique is problematic in that the pipesmust be heated to a considerably higher temperature than the temperaturerequired for pyrolysis in order to avoid inordinately long processingtimes. However, overheating of some of the oil shale is inefficient inthat it wastes input electrical energy, and may undesirably carbonizeorganic matter and decompose the rock matrix, thereby limiting theyield. Further electrical in situ techniques have been termed as "ohmicground heating" or "electrothermic" processes wherein the electricconductivity of the formations is relied upon to carry an electriccurrent as between electrodes placed in separated boreholes. An exampleof this type of technique, as applied to tar sands, is described in U.S.Pat. No. 3,848,671. A problem with this technique is that the formationsunder consideration are generally not sufficiently conductive tofacilitate the establishment of efficient uniform heating ourrents.Variations of the electrothermic techniques are known as"electrolinking", "electrocarbonization", and "electrogasification"(see, for example, U.S. Pat. No. 2,795,279). In electrolinking orelectrocarbonization, electric heating is again achieved via theinherent conductivity of the fuel bed. The electric current is appliedsuch that a thin narrow fracture path is formed between the electrodes.Along this fracture path, pyrolyzed carbon forms a more highlyconducting link between the boreholes in which the electrodes areimplanted. Current is then passed through this link to cause electricalheating of the surrounding formations. In the electrogasificationprocess, electrical heating through the formations is performedsimultaneously with a blast of air or steam. Generally, the justdescribed techniques are limited in that only relatively narrowfilament-like heating paths are formed between the electrodes. Since theformations are usually not particularly good conductors of heat, onlynon-uniform heating is generally achieved. The process tends to be slowand requires temperatures near the heating link which are substantiallyhigher than the desired pyrolyzing temperatures, with the attendantinefficiencies previously described.

Another approach to in situ processing has been termed"electrofracturing". In one variation of this technique, described inU.S. Pat. No. 3,103,975, conduction through electrodes implanted in theformations is again utilized, the heating being intended, for example,to increase the size of fratures in a mineral bed. In another version,disclosed in U.S. Pat. No. 3,696,866, electricity is used to fracture ashale formation and a thin viscous molten fluid core is formed in thefracture. This core is then forced to flow out to the shale by injectinghigh pressured gas in one of the well bores in which an electrode isimplanted, thereby establishing an open retorting channel.

In general, the above described techniques are limited by the relativelylow thermal and electrical conductivity of the bulk formations ofinterest. While individual conductive paths through the formations canbe established, heat does not radiate at useful rates from these paths,and efficient heating of the overall bulk is difficult to achieve.

A further proposed electrical in situ approach would employ a set ofarrays of dipole antennas located in a plastic or other dielectriccasing in a formation, such as a tar sand formation. A VHF or UHF powersource would energize the antennas and cause radiating fields to beemitted therefrom. However, at these frequencies, and considering theelectrical properties of the formations, the field intensity dropsrapidly as a function of distance away from the antennas. Therefore,once again, non-uniform heating would result in the need for inefficientoverheating of portions of the formations in order to obtain at leastminimum average heating of the bulk of the formations.

A still further proposed scheme would utilize in situ electricalinduction heating of formations. Again, the inherent (although limited)conduction ability of the formations is relied upon. In particular,secondary induction heating currents are induced in the formations byforming an underground toroidal induction coil and passing electricalcurrent through the turns of the coil. The underground toroid is formedby drilling vertical and horizontal boreholes and conductors arethreaded through the boreholes to form the turns of the toroid. It hasbeen noted, however, that as the formations are heated and water vaporsare removed from it, the formations become more resistive, and greatercurrents are required to provide the desired heating.

The above described techniques are limited by either or both of therelatively low thermal and electrical conductivity of the bulkformations of interest. Electrical techniques utilized for injectingheat energy into the formations have suffered from limitations givenrise to by the relatively low electrical conductivity of the bulkformations. In situ electrical techniques appear well capable ofinjecting heat energy into the formations along individual conductivepaths or around individual electrodes, but this leads to non-uniformheating of the bulk formations. The relatively low thermal conductivityof the formations then comes into play as a limiting factor in attaininga relatively uniformly heated bulk volume. The inefficiencies resultingfrom non-uniform heating have tended to render existing techniques slowand inefficient.

It is an object of the present invention to provide in situ heatprocessing of hydrocarbonaceous earth formations utilizing electricalexcitation means, in such a manner that substantially uniform heating ofa particular bulk volume of the formations is efficiently achieved.

Further objects of the present invention are to provide a system andmethod for efficiently heat processing relatively large blocks ofhydrocarbonaceous earth formations with a minimum of adverseenvironmental impact and for yielding a high net energy ratio of energyrecovered to energy expended.

SUMMARY OF THE INVENTION

Applicants have devised a technique for uniform heating of relativelylarge blocks of hydrocarbonaceous formations using radio frequency (RF)electrical energy that is substantially confined to the volume to beheated and effects dielectric heating of the formations. An importantaspect of applicants' invention relates to the fact that certainhydrocarbonaceous earth formations, for example raw unheated oil shale,exhibit dielectric absorption characteristics in the radio frequencyrange. As will be described, various practical constraints limit therange of frequencies which are suitable for the RF processing ofcommercially useful blocks of material in situ. The use of dielectricheating eliminates the reliance on electrical conductivity properties ofthe formations which characterize most prior art electrical in situapproaches. Also, unlike the proposed schemes which attempt to radiateelectrical energy from antennas in uncontrolled fashion, applicantsprovide field confining structures which maintain most of the inputenergy in the volume intended to be heated. Conduction currents, whichare difficult to establish on a useful uniform basis, are kept to aminimum, and displacement currents dominate and provide the desiredsubstantially uniform heating. Since it is not necessary for theresultant heat to propagate over substantial distances in the formations(as in the above described prior art ohmic heating schemes) therelatively poor thermal conductivity of the formations is not aparticular disadvantage in applicants' technique. Indeed, inalready-processed formations from which the useful products have beenremoved, the retained heat which is essentially "stored", can beadvantageously utilized. In an embodiment of the invention, initialheating of adjacent blocks of hydrocarbonaceous formations isimplemented using this retained heat.

In particular, the present invention is directed to a system and methodfor in situ heat processing of hydrocarbonaceous earth formations. Inaccordance with the system of the invention, a plurality of conductivemeans are inserted in the formations and bound a particular volume ofthe formations. As used herein, the phrase "bounding a particularvolume" is intended to mean that the volume is enclosed on at least twosides thereof. As will become understood, in the most practicalimplementations of the invention the enclosed sides are enclosed in anelectrical sense and the conductors forming a particular side can be anarray of spaced conductors. Electrical excitation means are provided forestablishing alternating electric fields in the volume. The frequency ofthe excitation means is selected as a function of the dimensions of thebound volume so as to establish a substantially non-radiating electricfield which is substantially confined in said volume. In this manner,volumetric dielectric heating of the formations will occur to effectapproximately uniform heating of the volume.

In the preferred embodiment of the invention, the frequency of theexcitation is in the radio frequency range and has a frequency betweenabout 1 HMz and 40 MHz. In this embodiment, the conductive meanscomprise opposing spaced rows of conductors disposed in opposite spacedrows of boreholes in the formations. One particularly advantageousstructure in accordance with the invention employs three spaced rows ofconductors which form a triplate-type of waveguide structure. The statedexcitation may be applied as a voltage, for example across differentgroups of the conductive means or as a dipole source, or may be appliedas a current which excites at least one current loop in the volume. Whena triplate-type of structure is employed, the conductors of the centralrow are preferably substantially shorter than the length of theconductors of the outer rows so as to reduce radiation, and resultantheat loss, at the ends of the conductors.

In accordance with a further feature of the invention, the frequency ofthe excitation is selected as a function of the electrical lossiness ofthe formations in the confined volume to be sufficiently low that the(l/e) attenuation distance of the electric field in any direction in thevolume is more than twice the physical dimension of the volume in thatdirection. In this manner, the diminution of the electric field in anydirection due to transfer of energy to the formations (as in, of course,desirable to effect the needed heating) is not so severe as to causeundue nonuniformity of heating in the volume and wasteful overheating ofportions thereof. As will be described, a further technique is employedfor obtaining relatively uniform heating by modifying the electric fieldpattern during the heating process so as to effectively average theelectric field intensity in the volume to enhance the uniformity ofheating of the volume.

The electrical heating techniques disclosed herein the applicable tovarious types of hydrocarbon-containing formations, including oil shale,tar sands, coal, heavy oil, partially depleted petroleum reservoirs,etc. The relatively uniform heating which results from the presenttechniques, even in formations having relatively low electricalconductivity and relatively low thermal conductivity, provides greatflexibility in applying recovery techniques. Accordingly, as will bedescribed, the in situ electrical heating of the present invention canbe utilized either alone or in conjunction with other in situ recoverytechniques to maximize efficiency for given applications.

Further features and advantages of the invention will become morereadily apparent from the following detailed description when taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an in situ twin lead transmission line in earthformations.

FIG. 2 illustrates an in situ biplate transmission line in earthformations.

FIG. 3 illustrates an in situ triplate transmission line in earthformations.

FIG. 4A is a plan view of an in situ structure in accordance with anembodiment of the invention.

FIG. 4B is an end view of the structure of FIG. 4A as taken through asection defined by arrows 4b--4b of FIG. 4A.

FIG. 4C is a side view of the structure of FIG. 4A as taken through asection defined by arrows 4c--4c of FIG. 4A.

FIG. 5 illustrates an alternate configuration of the structure of FIG.4B wherein the outer rows of conductors taper toward each other.

FIG. 6 illustrates implementation of the invention in a situation of amoderately deep resource bed.

FIG. 7 illustrates implementation of the invention in a situation wherea relatively thick resource bed is located relatively deep in theearth's surface.

FIG. 8 is a graph of the electric field and heating patterns resultingfrom a standing wave pattern in a triplate-type live configuration.

FIG. 9 illustrates a smoothly varying exponential heating pattern whichresults from modifying of the electric field pattern during operation.

FIG. 10 is a graph of operating frequency versus skin depth for an insitu oil shale heating system.

FIG. 11 is a graph of operating frequency versus processing time for anin situ oil shale heating system.

FIG. 12A illustrates an embodiment of the invention wherein current loopexcitation is employed.

FIG. 12B is an enlargement of a portion of FIG. 12A.

FIG. 13 is a simplified schematic diagram of a system and facility forrecovery of shale oil and related products from an oil shale bed.

FIG. 14 is a simplified schematic diagram of a system and facility forrecovery of useful constituents from a tar sand formation.

FIG. 15 is a simplified schematic diagram which illustrates how residualheat in "spent" formations can be utilized for pre-heating resources tobe subsequently processed.

FIG. 16 illustrates an embodiment of the invention wherein electricdipole excitation is employed.

FIG. 17 shows a diagram of a non-resonant processing technique.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Before describing the preferred implementations of practical forms ofthe invention, the principles of the invention can be initiallyunderstood with the air of the simplified diagrams of FIGS. 1, 2 and 3.FIG. 1 illustrates a twin-lead transmission line defined by a pair ofelongated conductors 101 and 102 which are inserted intohydrocarbonaceous earth formations 10, for example an oil shale or coaldeposit. A source 110 of radio frequency excitation is coupled to thetwin-lead transmission line. The resultant electric field causesheating, the heating being indicated in the FIGURES by the dots. Theintensity of the heating is represented by the density of the dots. InFIG. 1, the field lines, which are in a general standing wave pattern,extend well outside the region between the transmission line leads andsubstantial radiation occurs from various points with resultant loss ofheating control. (The actual field pattern will depend, inter alia, uponfrequency, as will be discussed below, and the illustrations of FIGS. 1,2 and 3 are for an appropriately chosen exemplary frequency.) In FIG. 2,there is illustrated a biplate transmission line consisting of spacedparallel conductive plates 201 and 202 in the formations. When excitedby a source 210 of RF energy, a standing wave field pattern is againestablished. Radiation is particularly prevalent at the edges andcorners of the transmission line plates. Radiation outside thetransmission line confined region is less than in FIG. 1, but stillsubstantial, as is evident from the heating pattern. FIG. 3 illustratesa triplate transmission line which includes spaced outer parallel plateconductors 301 and 302 and a central parallel plate conductor 303therebetween. Excitation by an RF source 310, as between the centralplate and the outer plate, establishes a fairly well confined field. Thecentral plate 303 is made shorter than the outer plates 301 and 302, andthis contributes to minimizing of fringing effects. Standing waves wouldalso normally be present (as in FIGS. 1 and 2) but, as will be describedfurther hereinbelow, the periodic heating effects caused by standingwave patterns can be averaged out, such as by varying the effectivelength of the center plate 303 during different stages of processing.The resultant substantially uniform average heating is illustrated bythe dot density in FIG. 3.

It is seen from the FIGS. 2 and 3 that alternating electric fieldssubstantially confined within a particular volume of hydrocarbonaceousformations can effect dielectric heating of the bulk material in thevolume. The degree of heating at each elemental volume unit in the bulkwill be a function of the dielectric lossiness of the material at theparticular frequency utilized as well as a function of the fieldstrength. Thus, an approximately uniform field in the confined volumewill give rise to approximately uniform heating within the volume, theheating not being particularly dependent upon conduction currents whichare minimal (as compared to displacement currents) in the presenttechniques.

As previously indicated, the illustrations of FIGS. 1, 2 and 3 areintended for the purpose of aiding in an initial understanding of theinvention. The structures of FIGS. 2 and 3, while being within thepurview of the invention, are not presently considered as preferredpractical embodiments since plate conductors of large size could not bereadily inserted in the formations. As will become understood, theconfining structures of FIGS. 2 and 3 can be approximated by rows ofconductors which are inserted in boreholes drilled in the formations.

One preferred form of applicants' invented system and method isillustrated in conjunction with FIGS. 4A, 4B and 4C. FIG. 4A shows aplan view of a surface of a hydrocarbonaceous deposit having three rowsof boreholes with elongated conductors therein. This structure is seento be analagous to the one in FIG. 3, except that the solid parallelplate conductors are replaced by individual elongated tubular conductorsplaced in boreholes that are drilled in relatively closely spacedrelationship to form outer rows designated as row 1 and row 3, and acentral row designated as row 2. The rows are spaced relatively farapart as compared to the spacing of adjacent conductors of a row. FIG.4B shows one conductor for each row; viz., conductor 415 from row 1,conductor 425 from row 2, and conductor 435 from row 3. FIG. 4Cillustrates the conductors of the central row, row 2. In the embodimentshown, the boreholes of the center row are drilled to a depth of L₁meters into the formations where L₁ is the approximate depth of thebottom boundary of the hydrocarbonaceous deposit. The boreholes of theouter rows are drilled to a depth of L₂, which is greater than L₁ andextends down into the barren rock below the useful deposit. Afterinserting the conductors into the boreholes, the conductors of row 2 areelectrically connected together and coupled to one terminal of an RFvoltage source 450 (see FIG. 4B). The conductors of the outer rows arealso connected together and coupled to the other terminal of the RFvoltage source 450. The zone heated by applied RF energy isapproximately illustrated by the cross-hatching of FIG. 4A. Theconductors provide an effective confining structure for the alternatingelectric fields established by the RF excitation. As will becomeunderstood, heating below L₁ is minimized by selecting the frequency ofoperation such that a cutoff condition substantially preventspropagation of wave energy below L₁.

The use of an array of elongated cylindrical conductors to form a fieldconfining structure is advantageous in that installation of these unitsin boreholes is more economical than, for example, installation ofcontinuous plane sheets on the boundaries of the volume to be heated insitu. Also, enhanced electric fields in the vicinities of the boreholeconductors, through which recovery of the hydrocarbonous fluidsultimately occurs, is actually a benefit (even though it represents adegree of heating non-uniformity in a system where even heating isstriven for) since the formations near the borehole conductors will beheated first. This tends to create initial permeability, porosity andminor fracturing which facilitates orderly recovery of fluids as theoverall bound volume later rises in temperature. To achieve fieldconfinement, the spacing between adjacent conductors of a row should beless than about a quarter wavelength apart and, preferably, less thanabout an eighth of a wavelength apart.

Very large volumes of hydrocarbonaceous deposits can be heat processedusing the described technique, for example volumes of the order of 10⁵cubic meters of oil shale. Large blocks can, if desired, be processed insequence by extending the lengths of the rows of boreholes andconductors. Alternative field confining structures and modes ofexcitation are possible and will be described further hereinbelow. Atpresent, however, two alternatives will be mentioned. First, furtherfield confinement can be provided by adding conductors in boreholes atthe ends of the rows (as illustrated by the dashed boreholes 490 of FIG.4A) to form a shielding structure. Secondly, consider the configurationof FIG. 5 (analagous to the cross-sectional view of FIG. 4B) wherein theconductors of the outer rows are tapered toward the central rows attheir deep ends so as to improve field uniformity (and consequently,heating uniformity) further from the source.

FIGS. 1-5 it was assumed, for ease of illustration, that thehydrocarbonaceous earth formations had a seam at or near the surface ofthe earth, or that any overburden had been removed. However, it will beunderstood that the invention is equally applicable to situations wherethe resource bed is less accessible and, for example, underground miningis required. In FIG. 6 there is shown a situation wherein a moderatelydeep hydrocarbonaceous bed, such as an oil shale layer of substantialthickness, is located beneath barren rock formations. In such instance,a drift or adit 640 can be mined and boreholes can be drilled from thesurface, as represented by the boreholes 601, 602 and 603 of FIG. 6, orfrom the drift. Again, each of these boreholes represents one of a rowof boreholes for a triplate-type configuration as is shown in FIG. 4.After the boreholes have been drilled, tubular conductors 611, 612 and613 are respectively lowered into the lower borehole portions in theresource bed. The coaxial lines 660 carrying the RF energy from a source650 to the tubular conductors can now be strung down an upper portion ofone or more of the boreholes and then connected across the differentrows of tubular conductors at drift 640. In this manner, there is nosubstantial heating of the upper barren rock as might be the case if theconductors were coupled from the surface of each borehole.

FIG. 7 illustrates a situation wherein a relatively thinhydrocarbonaceous deposit is located well below the earth's surface. Insuch case, a drift or adit 640 is first provided, and horizontalboreholes are then drilled for the conductors. The FIG. 7 againillustrates a tri-plate type configuration of three rows of boreholes,with the conductors 701, 702 and 703 being visible in the FIGURE.

The selection of suitable operating frequencies in the present inventiondepends upon various factors which will now be described. As radiofrequency (RF) electromagnetic wave energy propagates within thehydrocarbon-bearing media of interest, electrical energy is continuouslyconverted to heat energy. The two primary energy conversion mechanismsare ohmic heating, which results from the conductivity of theformations, and dielectric heating, which results from rotation ofmolecular dipoles by the alternating electric field of the wave energy.At any elemental volume point, x, within the formations of interest, thedielectric permittivity at a frequency f can be expressed as

    ε(x,f)=[ε.sub.r '(x,f)-jε.sub.r "(x,f)ε.sub.o                                     (1)

where ε_(r) '(x,f) is the relative real part of the complex dielectricpermittivity, ε_(r) "(x,f) is the relative imaginary part of thedielectric permittivity and represents both conductivity and dielectriclosses and ε_(o) is the permittivity of free space. The heating powerdensity, U(x,f) at point x can be expressed as

    U(x,f)=πfε.sub.r "(x,f)ε.sub.o E.sup.2 (x) watts/meter.sup.3                                         (2)

where E(x) is the electric field intensity at the point x. At radiofrequencies (0.3 MHz. to 300 MHz.) dielectric heating predominates forthe types of formations of interest herein, and the shale, tar sand, andcoal desposits to be treated can be considered as "lossy dielectrics".

As the electromagnetic wave energy is converted to heat, the electricfield wave progressively decays in exponential fashion as a function ofdistance along the path of the wave propagation. For each electricalskin depth, Δ, that the wave traverses, there is a reduction in the waveelectric field by about 63%. The skin depth, Δ, is related to thepropagation medium's permittivity and the electromagnetic wave frequencyby the relationship ##EQU1##

The heating resulting from electromagnetic waves in hydrocarbon-bearingformations diminishes progressively as the wave energy penetratesfurther into the formations and away from the source thereof. Thus, theuse of RF energy does not, per se, yield uniform heating of theformations of interest unless particular constraints are applied in theselection of frequency and field confining structure.

An idealized in situ heating technique would elevate all points withinthe defined heating zone to the desired processing temperature and leavevolumes outside the heating zone at their original temperature. Thiscannot be achieved in practice, but a useful goal is to obtainsubstantially uniform final heating of the zone, e.g. temperatures whichare within a ±10% range throughout. Since the heating power density,U(x,f), is a function of the square of the electric field intensity, E,it is desirable to have E within the range of about ±5% of a given levelin most of the processing zones. Consider, for example, the triplateline structure of FIG. 4 as being imbedded in an oil shale formation. Anelectromagnetic wave is excited by the RF power source 450 at thesurface of the oil shale seam and propagates down the triplate line intothe shale. The wave decays exponentially with distance from the surfacebecause of conversion of electrical energy into heat energy. Uponreaching the end of the center conductor, at a depth of L₁ meters, it isdesired that the wave undergo substantially total reflection. This isachieved by selecting the excitation frequency such that the halfwavelength λ_(l) /2 along the tri-plate line is substantially greaterthan the spacing between the outer rows, thereby giving rise to a cutoffcondition.

The result of the wave attenuation and reflection is the generation of astanding wave along the length of the triplate line. At a point, x, onthe line, the magnitude of the total standing wave electric field, E_(T)--x, from the end of the center conductor is ##EQU2## where Δ_(l) is theelectrical skin depth for a wave traveling along the triplate line, andλ_(l) is the wavelength along the triplate line, (Δ_(l) and λ_(l) beingassumed constant along the length of the line.)

To illustrate the nature of the standing wave pattern and heatingpotential resulting from the triplate-type line of structure of FIG.(4), equation (4) can be used to compute the ratios E_(T) (x)/E_(T) (O)and U(x)/U(O)=[E_(T) (x)/E_(T) (O)]² for the triplate line. Typicalresults are shown in the graph of FIG. 8. It is seen that E_(T) and Udecay with depth and exhibit an oscillatory behavior near L₁, withinterleaved peaks and nulls separated by a constant distance, λ_(l) /4,from each other. The position of the deepest peak coincides with the endof the center conductor at L₁ ; the position of the deepest null is atL₁ --λ_(l) /4.

An in situ triplate-type of structure having a heating potentialdistribution as shown in FIG. 8 will more easily meet heating uniformitygoals over its length if the oscillatory pattern could be smoothed out.This can be done by modifying the electric field pattern so as toeffectively average the electric field intensity in the volume beingheated. This may be achieved by physically decreasing the insertiondepth of the center conductor by λ_(l) /4 units midway through theheating time. Pulling each tube of the center conductor λ_(l) /4 unitsout of its respective borehole, or employing small explosive charges tosever the deepest λ_(l) /4 units of each tube are two ways this can bedone. Shifting the end of the center conductor in this manner wouldshift the entire standing wave pattern toward the surface of the oilshale seam by a distance of λ_(l) /4 units. Thus, heating peaks would bemoved to the positions of former heating nulls, and vice versa. Averagedover the entire heating time, the spatially oscillatory behavior of Uwould largely disappear. This can be demonstrated mathematically usingequations (2) and (3): ##EQU3## where K is a constant set by the powerlevel of the source. Equation (5) represents a smoothly varyingexponentially decreasing distribution of U, as shown in FIG. 9. It willbe understood that electrical means could alternatively be utilized tomodify the electric field pattern so as to average the electric fieldintensity in the volume being heated. Modification of the phase orfrequency of the excitation could also be employed.

The described technique of effectively averaging the electric fieldsubstantially eliminates peaking-type heating non-uniformities, but itis seen that the exponential decay of the electric field still posesdifficulties in attaining substantially uniform heating. In order tominimize the latter type of heating non-uniformity, the frequency ofoperation is selected such that the (l/e) attenuation distance Δ_(l) isgreater than the length L₁ and, preferably, greater than twice thelength L₁.

The value of Δ_(l) which is allowable for a particular heatinguniformity criterion can be determined from equation (5) by setting theheating potential at x=L₁ --λ_(l) /4 (the final position of the end ofthe center conductor) to be a desired percentage of the heatingpotential at x=0. For example, a heating goal of ±10% in the volumewould indicate that the desired percentage is 80%, so we have: ##EQU4##assuming the ε"(L₁ --λ_(l) /4)=ε"(0). For the present situation, thefollowing inequalities hold true:

    λ.sub.l /4<<Δ.sub.l,λ.sub.l /4<<L.sub.1. (7)

Using these inequalities, equation (6) can be rewritten as: ##EQU5## orequivalently as:

    sinh.sup.2 (L.sub.1 /Δ.sub.l)≈0.125,         (9)

which has the solution

    L.sub.1 =L.sub.1max ≈0.35 Δ.sub.l.           (10)

Thus, the length of the center conductor row of the triplate-type lineshould not exceed 35% of the line (l/e) attenuation distance in order toinsure heating uniformity within ±10% over the length of the line.Stated another way, to meet this heating uniformity requirement thefrequency of excitation should be sufficiently low to insure a skindepth of about three times L₁.

For an in situ triplate line type of structure (e.g. FIG. 4) with noartificial loading by either lumped capacitances or inductances, theexpression for Δ is given by (3) above, and combining (3) and (10)gives: ##EQU6##

To determine the variation of L₁.sbsb.max with frequency for oil shale,laboratory tests were conducted to obtain the electrical permittivity ofdry, Mahogany-type, Colorado oil shale over the frequency range of 1 MHzto 40 MHz. Using the data in conjunction with equations (3) and (11)curves for Δ and L₁.sbsb.max were plotted versus frequency, as shown inFIG. 10. It is seen, for example, that to allow the use of a singletriplate-type structure to process in situ a complete top to bottomsection of an oil shale bed with a thickness of 100 meters, the maximumoperating frequency which meets the stated heating uniformity criterionwould be 18 MHz. In a similar manner, FIG. 9 can be used to determinethe maximum operating frequency for triplate-type structures used toheat process shale beds ranging in thickness from 10 meters (f_(max) =95MHz) to 2500 meters (f_(max) =1 MHz). It will be understood thattrade-offs as between line length and frequency can be effected when,for example, it is desirable to select a particular frequency to complywith government radio frequency interference requirements.

Capacitive loading could also be employed to minimize amplitudereduction effects. For example, series capacitors can be inserted atregular intervals along the tubes of the center conductor of thetriplate line. These capacitors would act to partially cancel theeffective series inductance of the center conductor. Using theexpression for Δ_(l) of an arbitrary lossy transmission line, it can beshown that ##EQU7## for an in situ triplate-type line, where Δ is thenominal (l/e) attenuation distance at the operating frequency, and r isthe percentage reduction of the center conductor inductance caused bythe inserted capacitors. For example, if the effective center conductorinductance were reduced by 75%, Δ_(l) would increase by 100% to a valueof 2Δ.

Having set forth considerations which are used in determining maximumoperating frequency, attention is now turned to the selection ofsuitable minimum operating frequency.

The rate of resource heating is controlled by U(x,f), the heating powerdensity generated by the electromagnetic field. As seen fromrelationship (2), there are two types of factors influencing the rate ofheating: a frequency-independent amplitude factor, E² (x); and afrequency-dependent factor, fε" (x,f). To achieve rapid heating of theresource body, it would be desirable to generate a large value of E.However, if E is increased beyond some maximum value, designatedE_(max), the RF electric field could cause arc-over or breakdown of therock matrix and carbonized, conducting paths might form between theinner and outer conductors of the in situ confining structure. Thiscould lead to undesirable short circuiting of the system. To avoid thispossibility, the average RF electric field within the structure isconstrained to be no more than (S)E_(max), where S is a dimensionlesssafety factor in the range 0.01-0.1. In this way, reliable operation isinsured despite electric field enhancement at the surfaces of theconducting tubes of the FIG. 4 structure and possible local variationsof the breakdown level of the resource. A pilot or demonstration scaleRF in situ facility could operate with a typical S factor close to 0.1so that simulated production runs could be completed rapidly. However, alarge scale, commercial facility would likely be designated moreconservatively, i.e., with an S factor close to 0.01, to assure normaloperation of an associated high power RF generator under "worst case"conditions. Using E_(avg). =SE_(max) in relationship (2) yields

    U.sub.average (f)≦S.sup.2 ·[πfε.sub.r "(f)ε.sub.o E.sup.2.sub.max ]W/m.sup.3            (13)

The RF heating power density varies as the square of S, so selection ofS has an important impact on the processing time and, as will be seen,selection of minimum operating frequency. It is seen from relationships(2) and (13) that increasing the product term, fε_(r) "(x,f), increasesthe electromagnetic heating power density regardless of the electricfield amplitude. This product term is found to increase monotonically inthe frequency range of 1 MHz to 40 MHz for oil shale. Thus, for a givenRF electric field, increasing the operating frequency causes the shaleheating rate to increase. Considerations of maximum operating frequency,set forth above, must be borne in mind, however.

The minimum processing time at a particular operating frequency, t_(min)(f), can be derived as a function of the fraction, R, of spent shalesensible heat that can be recycled (this aspect to be treated below),the RF electric field breakdown level, E_(max), of the shale rock, thesafety factor, S, and the loss component, ε_(r) " (f), of the shale.First, the total RF heating energy required to process one cubic meterof raw oil shale can be calculated, assuming an oil shale density of 1.6g/cm³ (1.6·10³ kg/m³) and assuming ##EQU8## Now, t_(min) (f) can befound by dividing the RF heating requirement of Equation (14a) by themaximum RF heating power density of Equation (13): ##EQU9##

FIG. 11 uses Equation (14b) to plot versus frequency the minimumprocessing time (with S=0.01 and S=0.1) for RF heating of dry,Mahogany-type Colorado oil shale. It is assumed that E_(max) =10⁶ V/mand is independent of the operating frequency, and the R=0.5. From FIG.11, it is seen that, for S=0.1, t_(min) ranges from 0.6 hours at 40 MHzto 36 hours at 1 MHz, and to an extrapolated time of about 300 hours at0.1 MHz. For S=0.01 t_(min) ranges from 60 hours at 40 MHz to 3600 hours(5 months) at 1 MHz.

During the processing cycle of a block of shale using the presenttechnique, heat conduction to adjacent shale regions can tend to degradethe desired heating uniformity by causing cooling of the boundary planesof the shale block being processed. Further, such thermal conductionresults in heat energy flowing outside the block of interest,complicating the problem of controlling the extent and efficiency of theheating process. Such an outflow of heat further increases the necessaryheating time. Actual determination of heat flow effects is a complexfunction of the size and shape of the shale blocks being heated;however, an illustration of such effects on the graphs of FIG. 11 isdepicted by the dotted lines curves for a hypothetical block of shale.

In order to limit these undesired consequences of resource heatconduction, it is desirable to complete the processing cycle of theblock being treated before appreciable heat energy can flow out of theblock. Based on these considerations, applicants have selected a maximumelectrical processing time of about two weeks, with preferred processingtimes being less than this time. From FIG. 11, this condition would meanthat the operating frequency could be no lower than 0.1 MHz for theS=0.1 case, and could be no lower than 10 MHz for the S=0.01 case. Anintermediate value of S would accordingly yield an intermediate "orderof magnitude" frequency of 1 MHz. The frequency lower bound (based onconsiderations of heat conduction away from the electrically heated zoneand conservative design relative to shale breakdown) can be combinedwith the frequency upper bound obtainable from FIG. 10 (based onconsiderations of heating uniformity within the zone and shale skindepth) to define the preferred frequency range. For blocks ofcommercially practical size, a maximum frequency of about 40 MHz ispreferred, so that preferred frequency range is about 1 MHz to 40 MHz.It should be noted that other confining structures within the purview ofthe invention, such as waveguides and cavities, will have somewhatdifferent optimum operating frequency ranges because of differences inthe electromagnetic field patterns and heat conduction times peculiar toa given geometry.

It will be understood that there are other possible techniques forexciting the alternating electric field patterns to obtain dielectricheating of the formations bound by the confining conductor structures ofthe invention: i.e., alternative to the previously described techniqueof applying voltages across different groups of the conductors. In FIG.12 there is again shown a triplate-type of configuration having rows ofconductors designated as row 1, row 2 and row 3, the conductors againbeing inserted in boreholes drilled into hydrocarbonaceous formationssuch as an oil shale bed. In the embodiment of FIG. 12, the desiredfield pattern in the confined volume of formations is established usinga current loop excitation.

The conductors of the central row have loop exciters 121 and 122 formedintegrally therewith, the loop exciters 121 providing magnetic fieldexcitation to the left of the central row conductors and the loopexciters 122 providing magnetic field excitation to the right of thecentral row conductors. The established alternating electric fieldpattern, concomitant with the varying magnetic field, providessubstantially uniform dielectric heating in the manner previouslydescribed. The conductors of the central row have an outer tubular metalshell 123 and an inner conductor 124, shown in dashed line in FIG. 4A.Slots 125 and 126 are formed in the outer tube and the loops 121 and 122extend from the inner conductor, through the slots, and then reconnectwith the outer conductor as shown by the dashed line. The lower portion120 of the central row conductor extends from the bottom of the loop.

In operation, an RF current source 127 is coupled between the outertubular conductor 123 and the inner conductor 124 and drives currentthrough the loop 121 and 122, thereby establishing alternating magneticfields and concomitant electric fields which are confined in the volumebound by the rows of conductors in row 1 and row 3. A quarter wave stub128 is provided at about the top of the hydrocarbonaceous deposit and,in effect, creates an open circuit which isolates the conductor passingthrough the overburden from the lower portion thereof. This techniqueprevents energy from propagating back toward the source and heating theoverburden. Considerations of frequency are similar to those discussedabove. An advantage of the approach of FIG. 12 is that the voltagecarrying capability of the cables can be reduced since the possibilityof a voltage breakdown is diminished when using a current drive scheme.

It will be understood that various alternate techniques for excitationof the electric fields can be implemented to obtain dielectric heatingas defined herein. For example, electric dipole excitation could beemployed to generate the electric fields in the confined volume, so longas the previously described frequency limitations are met forestablishing relatively uniform dielectric heating. FIG. 16 illustratesan arrangement wherein electric dipole excitation is used. Centerconductor 166 is coupled to electrodes 166A and 166B which protrude fromslots in outer conductor 163, and a voltage source 167 is coupledbetween the inner and outer conductors.

In the configuration of FIG. 12, wherein a current loop drive isutilized, it is advantageous to use a source position which results inan odd number of quarter wavelengths from the position of the currentloop to each end of the central conductor, since the source is at avoltage minimum and it is desirable to have voltage maxima at the opencircuited terminations to achieve a resonance condition. Similarly, inFIG. 16 the dipole source is preferably located an even number ofquarter wavelengths from the ends of the central conductor.

Referring to FIG. 13, there is shown a simplified schematic diagram of asystem and facility for recovery of shale oil and related products froman oil shale bed. A tri-plate-type configuration of the naturepreviously described is used in this system. Three rows of boreholes,designated as row 1, row 2 and row 3, are drilled through the overburdenand into the oil shale bed, the central row of boreholes preferablybeing of a lesser depth than the outer rows. A drift 131 is mined in theoverburden above the oil shale formation so that electrical connectionscan be made in the manner described in conjunction with FIG. 6. Tubularconductors are inserted into the lower portions of the boreholes of eachrow. An RF source 132 is provided and obtains its power from a suitablepower plant which may or may not be located at the site. For ease ofillustration, the electrical connections are not shown in FIG. 13, butthey may be the same as those of FIG. 6. A network of pipes forinjection of suitable media are provided, the horizontal feed pipes 133,134 and 135 being coupled to the boreholes of row 1, row 2 and row 3,respectively, and suitable valves and cross-couplings also beingprovided. The art of injecting suitable media and recovering subsurfacefluids is well developed and not, taken alone, the subject of thisinvention, so the description thereof is limited to that necessary foran understanding of the present system and techniques. Recovered fluidsare coupled to a main discharge pipe 136 and then to suitable processingplant equipment which is also well known in the art. Again, these wellknown techniques will not be described in full detail herein, but aconduit 137 represents the process of separation of shale oil vapor andhigh and low BTU gas, whereas the conduit 138 represents the processingof shale oil vapor, in well known manner, to obtain synthethic crude.The overall processing system of FIG. 13 will vary somewhat in itsstructure and use, depending upon which of the to-be-described versionsof the present technique are utilized to recover valuable constituentsfrom the oil shale bed.

It will be recognized that the heating can be advantageously performedto different degrees in order to implement useful extraction of theorganic resources from the formations. These techniques will also varywith the type of resource form which the fuel is being recovered. In thecase of oil shale, three versions of extraction techniques utilizing theinvention are set forth, although it will become clear that variationsor combinations of these techniques could be readily employed by thoseskilled in the art. The first version aims only for recovery of shaleoil and by-product gases that correspond to the recovery aims ofpreviously proposed in situ oil shale processing techniques. Electricalradio frequency energy is applied, for example using the system of FIG.13, to heat a relatively large block of oil shale in situ to above 500°C. As the temperature passes the point where inherent shale moistureflashes into steam, some fracturing, at least along bedding planes, willtypically be experienced. Additional interconnecting voids will alsoform within unfractured pieces of oil shale during pyrolysis in the400°-500° C. range. While substantially uniform heating is striven for,heating is not exactly uniform and the oil shale nearer the electrodeswill be heating slightly more rapidly than the shale further away. As aresult, permeability is progressively established outward from theelectrodes, permitting passage of shale oil vapors up the hollowelectrode tubes for collection. In the same way, the considerablequantity of hydrocarbon gases liberated at shale temperatures betweenabout 200° C. to 500° C. will pass to the surface via the tubes. At thesurface of the earth, the shale oil vapors and bi-product gases arecollected and processed using known techniques, as depicted broadly inFIG. 13. In this first version there is not necessarily any attempt toutilize the carbonaceous residue left in the spent shale formations.

Another in situ processing version which utilizes the electrical radiofrequency heating techniques of the invention would aim to increase theyield of useful products from the oil shale resource and to reduceprocess energy consumption by making full use of the unique attributesof the disclosed in situ heating technique. Since heating to relativelyprecise temperatures is possible with the invented technique, thissecond version would apply heating to about 425° C. to recover crackedkerogen in liquid form. In this manner, the substantial electric energyneeded to apply the additional heat to volatilize the shale oil productwould be saved.

In either version of the process, a relatively high degree of porosityand permeability will be present after removal of the liquid kerogen.Thus, if desirable, subsequent recovery of the carbonaceous residue onthe spent shale could be achieved by injection of steam and either airor oxygen to initiate a "water-gas" reaction. Upon injection, the steamand oxygen react with the carbonaceous residue to form a low BTU gaswhich is recovered and can be used, for example, for the hydrogenationof the raw shale oil, or for on-site generation of electric power. Thewater-gas reaction would also result in a higher spent shaletemperature, for example 600° C., than in the case of the firstprocessing version. This would be advantageous when techniques, such asthose described below in conjunction with FIGS. 15, 16, are employed forusing residual heat for preheating the raw shale in other blocks in theshale bed. An overall saving of electrical energy would thereby beachieved. The creation of shale permeability and wetability afterremoval of the liquid kerogen would also permit extraction, in situ, ofvarious coproducts such as aluminum hydroxide, nahcolite, uranium orrelated minerals present in the shale by leaching methods.

In a third processing version, the electrical heating techniques of theinvention are employed only to relatively lower temperatures, belowabout 200° C. to obtain fast fracturing of the shale by vaporization ofmoisture content, whereupon combustion or thermal in situ extractiontechniques can be used to obtain the useful products.

It will be understood that various "hybrid" extraction approaches, whichinclude the electrical heating techniques of this invention, can beemployed, depending upon the type of oil shale formations in aparticular region, availability of electrical energy, and other factorsrelating to costs. For example, the disclosed electrical radio frequencyheating techniques could be employed in either the middle rangetemperatures or to "top off" temperature distributions obtained by otherheating methods.

Applicants have observed that raw unheated tar sand, heavy oil matrices,and partially depleted petroleum deposits exhibit dielectric absorptioncharacteristics at radio frequencies which render possible the use ofthe present techniques for heating of such deposits (tar sands beinggenerally referred to hereafter, for convenience) so that bitumen can berecovered therefrom. Again, the relatively low electrical conductivityand relatively low thermal conductivity of the tar sands is not animpediment (as in prior art techniques) since dielectric heating isemployed. The selection of a suitable range of frequencies in the radiofrequency band is based on considerations that are similar to those setforth above. If the selected frequencies of operation are too high, thepenetration of energy into the deposit is too shallow (i.e., a smallskin depth, as discussed above) and relatively large volumes of in situmaterial cannot be advantageously processed due to largenon-uniformities of heating. On the other hand, if the frequency ofoperation is selected below a certain range, the absorption of energyper unit volume will be relatively low (since dielectric absorption isroughly proportional to frequency over the range of interest), so theamplitude of the electrical excitation must be made relatively large inorder to obtain the necessary heating to prevent processing times frombecoming inordinately long. However, practical considerations limit thedegree to which the applied excitation can be intensified without therisk of electrical breakdown. Thus, once a maximum excitation amplitudeis selected, the minimum frequency is a function of desired processingtime. Applicants have discovered that the dielectric absorptioncharacteristics of tar sands are generally in a range similar to thatdescribed above in conjunction with oil shale, but somewhat lowerfrequencies within the radio frequency range are anticipated. However,it will be understood that variations in the optimum frequencies willoccur for different types of mineral deposits, different confiningstructures, and different heating time objectives.

In FIG. 14 there is shown a simplified schematic diagram of a system andfacility for recovery and processing of bitumen from a subterranean tarsand formation. A triplate-type configuration is again utilized withthree rows of boreholes, designated at row 1, row 2 and row 3, beingdrilled or driven through the overburden and into the tar sandformation, as in FIG. 13. A drift 141 is mined in the overburden abovethe tar sand formation so that electrical connections can be made in themanner described in conjunction with FIG. 6. Again, tubular conductorsare inserted into the lower portions of the boreholes of each row. An RFsource 142 is provided and, as before, for ease of illustration, theelectrical connections are not shown in FIG. 14, although they may bethe same as those of FIG. 6. As in FIG. 13, a network of pipes forinjection of suitable drive media is provided, the horizontal feedpipes143 and 145 being coupled to the boreholes of row 1 and row 3,respectively, in this instance. Pipe 146 is the main collection pipe andsuitable valves and cross-couplings are also provided. In the presentinstance, after suitable heating of the resource, steam or hot chemicalsolutions can typically be injected into at least some of the boreholesand the hot mobile tars are forced to the surface for collection viacollection pipes 144 and 146 and collection tank 147. Subsequentprocessing of the recovered tars is a well developed art and will not bedescribed herein. In the illustration of FIG. 14, the boreholes of rows1 and 3 are utilized as "injection wells" and the boreholes of rows 2are used as "production wells", although it will be understood thatvarious alternate techniques can be used for bringing the heated tars tothe surface.

As in the case of oil shale, it will be recognized that electricalheating can be advantageously performed to different degrees in order toimplement useful extraction of the organic resources from the tar sandformations.

In a first version of the tar sand or heavy oil recovery technique,electrical heating is applied to reduce the viscosity of the in-placetars or heavy oils to a point where other known complementary processescan be employed to recover the in-place fuels. In such case, radiofrequency electrical energy can be applied to relatively uniformly heata block of tar sands to a temperature of about 150° C. This, in effect,produces a volume of low viscosity fluids in the tar sand matrix whichis effectively sealed around its periphery by the lower temperature(impermeable or less permeable) cooler tar sands. Simple gravity flowinto producer holes or a pressurized drive, consistent with FIG. 14, canbe used to force the low viscosity fluids to the surface using injectionof hot fluids.

In a second version of the technique, useful fuels are recovered fromtar sand and heavy oil deposits by partially or completely pyrolyzingthe tars in situ. Electrical radio frequency energy is applied inaccordance with the principles of the invention to heat a relativelylarge block of tar sand in situ to about 500° C. As the temperature ofthe tar sand increases above about 100° C., the inherent moisture beginsto change into steam. A further increase in temperature to around 150°C. substantially reduces the viscosity of in-place tars or heavy oils.As the pyrolysis temperature is approached, the higher volatiles areemitted until complete pyrolysis of the in-place fuels is accomplished.The tar sands nearest the electrodes will be heated slightly morerapidly than the tar sands farther away, so regions of relatively lowviscosity and high permeability will be progressively establishedoutward from the electrodes. This permits passage of the high volatilesand pyrolytic product vapors up the boreholes for collection with orwithout a drive. A variation of this second version would subsequentlyemploy a water gas process, as described above, to produce a low BTU gasfrom the remaining pyrolytic carbon. Also, simple combustion of carbonresidues can be utilized in order to recover residual energy in the formof sensible heat. It will be understood that various combinations orsequences of the described steps can be performed, as desired.

Referring to FIG. 15, there is shown a schematic diagram whichillustrates how residual heat in the "spent" formations from whichconstituents have already been extracted can be utilized for pre-heatingof the next block of the resource to be processed. After the boreholesare formed in the new zone to be heat processed, a system of pipes canbe utilized to carry steamwater mixtures which effectively transfersresidual heat from the just-processed zone to the next zone to beprocessed. In FIG. 15, the relatively cool raw resource bed to beprocessed is illustrated by the block 151, and the spent hot resource isrepresented by the block 152. The water pumped into the block 152 viapump 153 and feed pipe 157 becomes very hot steam which is circulatedthrough the pipes 159 to the block 151. The system is "closed loop" sothat after heat from the steam is expended in the block 151, it isreturned as cooler steam or condensate to the block 152 via return pipe158. It will be understood that the sequentially processed zones may beadjacent zones to take advantage of thermal flow outside a volume beingprocessed. In particular, heat which flows outside the volume beingprocessed, which might normally be wasted, can be utilized in preheatingzones to be subsequently processed. Thus, for example, rows definingzones in the formations being processed can alternate with and"sandwich" zones to bo subsequently processed so that heat which flowsout of the zones presently being processed can be, to a substantialextent, utilized later. This technique, along with the use of residualheat in the "spent" formations, as described in conjunction with FIG.15, can substantially reduce the amount of total input energy needed forheat processing.

The present invention allows maximum extraction of desired organicproducts while keeping pollution and waste accumulation to a minimum andstill being economically advantageous. Very little mining, if any, isrequired and the pollution and waste aspects of above ground retortingare, of course, absent. The invented technique compares most favorablywith those in situ techniques that require combustion, since thosetechniques necessarily produce hot flue gases that must be cleaned ofparticulates, sulfur, etc. before release into the invironment. Afurther advantage is a result of the relatively close control over theheating zone which is a feature of the present invention and greatlyreduces the possibility of uncontrolled in situ combustion which canhave adverse safety and/or environmental effects.

The invention has been described with reference to particularembodiments, but variations within the spirit and scope of the inventionwill occur to those skilled in the art. For example, the term"boreholes" as used herein is intended generically to include any typeof hole or slot in the formation formed by any suitable means such asmechanical or water-jet drilling, pile driving, etc., as well as formsof mining or excavation. Also, the field confining conductors of thepresent invention can be of any desired form, including meshes, straps,or flexible foils, and will depend, to some degree, upon the locationand exposure of the particular surface of the volume they confine.Further, it will be understood that in addition to the resonant TEM typeof lines described herein, the confining structure can also take theform of single-mode TE or TM in situ waveguides or multi-mode enclosedcavities. In both instances, standing-wave correction, as previouslydescribed, can be employed to substantially average over time theelectric field (and resultant heating) throughout the confined volume,both electrical and mechanical techniques being available as disclosedhereinabove. The excitation frequency can also be varied duringoperation. In the case of a cavity, appropriate drifts or adits can bemined to obtain access to drilling locations (e.g. as illustrated inFIG. 7) so that conductors can be positioned to define surfaces thatcompletely confine a volume to be heated. The resultant "in situ cavity"would be somewhat similar in operation to a microwave oven (but withradio frequency energy being utilized). Mode mixing can be achieved, forexample, by utilizing a multiplicity of electric and/or magnetic dipolesat different locations on the walls or within the cavity andsequentially exciting them to obtain different modes to achievesubstantially uniform heating of the confined volume. Alternatively,conductors can be inserted and withdrawn from a series of boreholes, aspreviously described. The cavity approach is advantageous due to theabsence of geometrical constraints pertaining to achieving cutoff ofpotentially radiating wave energy. This means that larger blocks of theresource can be processed at once.

Further, it will be understood that non-resonant confining structurescan be utilized, if desired. For example, FIG. 17 is a simplifieddiagram illustrating how a nonresonant confining structure can beutilized in conjunction with a "sandwich" type of processing techniquethat utilizes thermal flow from spent regions. Three "loops" designatedas loop 170A, 170B, and 170C, are illustrated, each loop including, forexample, a pair of tri-plate lines of the type illustrated in FIG. 4.However, in this instance the central row of each tri-plate line is notintentionally truncated. Instead, connecting lines designated byreference numerals 171A, 171B and 171C are employed, this being done byinserting appropriate horizontal conductors from a mined tunnel.Switches 181-187 are provided and are intially positioned as shown inFIG. 17. In operation, the loops are first connected in series and theswitch 181 is coupled to the RF source 179. Wave energy is introducedinto the first tri-plate line of loop 170A and travels around the loopand is then connected via switch 183 to loop 170B, and so on. Dielectricheating of the hydrocarbonaceous formations is achieved, with theelectric field being progressively attenuated. Accordingly, the loop170A is heated more than the loop 170B which is heated more than theloop 170C, etc. When the hydrocarbonaceous deposit of loop 170A has beenheated to a desired degree, switches 181 and 183 are switched so thatloop 170A is no longer energized and loop 170B is now heated to thegreatest extent. This procedure is continued until the alternate layersof hydrocarbonaceous formations are fully heated to the extent desired.After a suitable period of time, typically weeks or months, for the heatfrom the spent regions to transfer into the between-loop formations, thebetween-loop formations can be processed in similar manner.

As previously noted, the invention is applicable to various types ofhydrocarbonaceous deposits, and variations in technique, consistent withthe principles of the invention, will be employed depending upon thetype of resource being exploited. For example, in the case of coal, theelectrical properties of the material indicates that the lower portionof the radio frequency spectrum, for example of the order of 100 KHz,will be useful. Further, it will be understood that as heat processingof a particular resource progresses, the properties of the resource canchange and may render advantageous the modification of operatingfrequency for different processing stages.

Applicants have observed that the raw materials under consideration cantend to exhibit different dielectric properties at differenttemperatures. As a consequence, it may be desirable to modify electricalparameters to match the characteristics of the AC power source to thecharacteristics of the field exciting structure whose properties areinfluenced by the different dielectric properties of the raw materials.A variable matching network, such as is represented by block 451 (indashed line) of FIG. 4A, can be used towards this end.

We claim:
 1. A system for in situ heat processing of hydrocarbonaceousearth formations, comprising:a plurality of conductive means inserted insaid formations and bounding a particular volume of said formations;electrical excitation means for establishing alternating electric fieldsin said volume; the frequency of said excitation means being selected asa function of the volume dimensions so as to establish substantiallynon-radiating electric fields which are substantially confined in saidvolume; whereby volumetric dielectric heating of the formations willoccur to effect approximately uniform heating of said volume.
 2. Asystem as defined by claim 1 wherein the frequency of said excitation isin the radio frequency range.
 3. A system as defined by claim 2 whereinsaid conductive means comprise opposing spaced rows of conductorsdisposed in opposing spaced rows of boreholes in said formations.
 4. Asystem as defined by claim 3 wherein the conductors of each row comprisespaced elongated conductors.
 5. A system as defined by claim .[.4.]..Iadd.20 .Iaddend.wherein said excitation is applied as a voltage asbetween the conductors of the outer rows and the conductors of thecentral row.
 6. A system as defined by claim 4 wherein said electricalexcitation is a source of current applied to at least one current loopin said volume.
 7. A system as defined by claim 4 wherein saidelectrical excitation is applied across at least one electrical dipolein said volume.
 8. A system as defined by claim 4 wherein the conductorsof the central row are of substantially shorter length than theconductors of the outer rows so as to reduce radiation at the ends ofsaid conductors.
 9. A system as defined by claim 8 wherein the frequencyof said excitation is selected such that a half wavelength ofelectromagnetic energy in the region beyond the center conductor issubstantially greater than the spacing between the outer rows to giverise to a cutoff condition in said region.
 10. A system as defined byclaim 9 wherein the frequency of said excitation is selected as afunction of the electrical lossiness of the formations in said volume tobe sufficiently low such that the l/e attenuation distance of theelectric field in any direction in said volume is more than twice thephysical dimension of said volume in that direction.
 11. A system asdefined by claim 9 further comprising means for modifying the electricfield pattern so as to average the electric field intensity in saidvolume to enhance the uniformity of heating of said volume.
 12. A systemas defined by claim 8 wherein the frequency of said excitation isselected as a function of the electrical lossiness of the formations insaid volume to be sufficiently low such that the l/e attenuationdistance of the electric field in any direction in said volume is morethan twice the physical dimension of said volume in that direction. 13.A system as defined by claim 8 further comprising means for modifyingthe electric field pattern so as to average the electric field intensityin said volume to enhance the uniformity of heating of said volume. 14.A system as defined by claim 2 wherein said excitation in applied as toa voltage as between different groups of said conductive means.
 15. Asystem as defined by claim 2 wherein said electrical excitation is asource of current applied to at least one current loop in said volume.16. A system as defined by claim 2 wherein said electrical excitation isapplied across at least one electrical dipole in said volume.
 17. Asystem as defined by claim 2 wherein the frequency of said excitation isselected as a function of the electrical lossiness of the formations insaid volume to be sufficiently low such that the l/e attenuationdistance of the electric field in any direction in said volume is morethan twice the physical dimension of said volume in that direction. 18.A system as defined by claim 2 further comprising means for modifyingthe electric field pattern so as to average the electric field intensityin said volume to enhance the uniformity of heating of said volume. 19.A system as defined by claim 1 wherein said conductive means compriseopposing spaced rows of conductors disposed in opposing spaced rows ofboreholes in said formations.
 20. A system as defined by claim 19wherein said rows of conductors comprise three spaced rows ofconductors.
 21. A system as defined by claim 20 wherein the conductorsof each row comprise spaced elongated conductors.
 22. A system asdefined by claim 21 wherein said excitation is applied as a voltage asbetween the conductors of the outer rows and the conductors of thecentral row.
 23. A system as defined by claim 22 wherein the conductorsof the central row are of substantially shorter length than theconductors of the outer rows so as to reduce radiation at the ends ofsaid conductors.
 24. A system as defined by claim 23 wherein thefrequency of said excitation is selected such that a half wavelength ofelectromagnetic energy in the region beyond the center conductor issubstantially greater than the spacing between the outer rows to giverise to a cutoff condition in said region.
 25. A system as defined byclaim 21 wherein said electrical excitation is a source of currentapplied to at least one current loop in said volume.
 26. A system asdefined by claim 25 wherein the conductors of the central row are ofsubstantially shorter length than the conductors of the outer rows so asto reduce radiation at the ends of said conductors.
 27. A system asdefined by claim 26 wherein the frequency of said excitation is selectedsuch that a half wavelength of electromagnetic energy in the regionbeyond the center conductor is substantially greater than the spacingbetween the outer rows to give rise to a cutoff condition in saidregion.
 28. A system as defined by claim 21 wherein said electricalexcitation is applied across at least one electrical dipole in saidvolume.
 29. A system as defined by claim 20 wherein the frequency ofsaid excitation is selected as a function of the electrical lossiness ofthe formations in said volume to be sufficiently low such that the l/eattenuation distance of the electric field in any direction in saidvolume is more than twice the physical dimension of said volume in thatdirection.
 30. A system as defined by claim 29 wherein said rows ofconductors are inserted in said formations at angles such that said rowsare closer together at far ends thereof to compensate for attenuation ofthe electrical field at said far end.
 31. A system as defined by claim20 further comprising means for modifying the electric field pattern soas to average the electric field intensity in said volume to enhance theuniformity of heating of said volume.
 32. A system as defined by claim31 wherein said means for modifying the electric field pattern comprisesmeans for modifying the effective length of the conductors of thecentral row.
 33. A system as defined by claim 32 wherein said means formodifying the effective length of the conductors of the central rowcomprises means for physically shortening the length of said conductors.34. A system as defined by claim 32 wherein said means for modifying theeffective length of said conductors comprises means for electricallymodifying the effective length thereof.
 35. A system as defined by claim20 wherein said rows of conductors are inserted in said formations atangles such that said rows are closer together at far ends thereof tocompensate for attenuation of the electrical field at said far end. 36.A system as defined by claim 19 wherein the frequency of said excitationis selected as a function of the electrical lossiness of the formationsin said volume to be sufficiently low such that the l/e attenuationdistance of the electric field in any direction in said volume is morethan twice the physical dimension of said volume in that direction. 37.A system as defined by claim 19 further comprising means for modifyingthe electric field pattern so as to average the electric field intensityin said volume to enhance the uniformity of heating of said volume. 38.A system as defined by claim 19 wherein said rows of conductors areinserted in said formations at angles such that said rows are closertogether at far ends thereof to compensate for attenuation of theelectrical field at said far end.
 39. A system as defined by claim 1wherein said excitation is applied as a voltage as between differentgroups of said conductive means.
 40. A system as defined by claim 39wherein the conductors of the central row are of substantially shorterlength than the conductors of the outer rows so as to reduce radiationat the ends of said conductors.
 41. A system as defined by claim 1wherein said electrical excitation is a source of current applied to atleast one current loop in said volume.
 42. A system as defined by claim1 wherein said electrical excitation is applied across at least oneelectrical dipole in said volume.
 43. A system as defined by claim 1wherein the frequency of said excitation is selected as a function ofthe electrical lossiness of the formations in said volume to besufficiently low such that the l/e attenuation distance of the electricfield in any direction in said volume is more than twice the physicaldimension of said volume in that direction.
 44. A system as defined byclaim 43 further comprising means for modifying the electric fieldpattern so as to average the electric field intensity in said volume toenhance the uniformity of heating of said volume.
 45. A system asdefined by claim 1 further comprising means for modifying the electricfield pattern so as to average the electric field intensity in saidvolume to enhance the uniformity of heating of said volume.
 46. A methodfor in situ heating of hydrocarbonaceous earth formations, comprisingthe steps of:forming a plurality of boreholes which bound a particularvolume of said formations; inserting elongated electrical conductors insaid boreholes; and introducing electrical excitation to said formationsto establish alternating electric fields in said volume; the frequencyof said excitation being selected as a function of the volume dimensionsso as to establish substantially non-radiating electric fields which aresubstantially confined in said volume; whereby volumetric dielectricheating of the formations will occur to effect approximately uniformheating of said volume.
 47. A method as defined by claim 46 wherein thefrequency of said excitation is in the radio frequency range.
 48. Amethod as defined by claim 47 wherein the step of introducing electricalexcitation comprises applying a voltage as between different groups ofsaid conductors.
 49. A method as defined by claim 47 wherein the step ofintroducing electrical excitation comprises applying electrical currentto at least one current loop in said volume.
 50. A method as defined byclaim 47 wherein the frequency of said excitation is selected as afunction of the electrical lossiness of the formations in said volume tobe sufficiently low such that the l/e attenuation distance of theelectric field in any direction in said volume is more than twice thephysical dimension of said volume in that direction.
 51. A method asdefined by claim 47 further comprising the step of modifying theelectric field pattern so as to average the electric field intensity insaid volume to enhance the uniformity of heating of said volume.
 52. Amethod as defined by claim 51 wherein the step of modifying the electricfield pattern comprises the step of modifying the effective length ofsome of said conductors.
 53. A method as defined by claim 47 furthercomprising the step of withdrawing through said boreholes the valuableconstituents resulting from said heating.
 54. A method as defined byclaim 47 wherein said dielectric heating is continued to heat saidvolume to a temperature below the temperature required for extraction ofvaluable constituents from said volume, and further comprising the stepsof applying further nonelectrical heating means to said volume andwithdrawing through said boreholes valuable constituents from saidvolume.
 55. A method as defined by claim 46 wherein said boreholes areformed in opposing spaced rows in said formations.
 56. A method asdefined by claim 55 wherein said rows comprise three spaced rows.
 57. Asystem for in situ heat processing of an oil shale bed, comprising:aplurality of conductive means bounding a particular volume of said bed;electrical excitation means for establishing alternating electric fieldsin said volume; the frequency of said excitation means being selected asa function of the volume dimensions so as to establish substantiallynon-radiating electric fields which are substantially confined in saidvolume; whereby volumetric dielectric heating of the bed will occur toeffect approximately uniform heating of said volume.
 58. A system asdefined by claim 57 wherein the frequency of said excitation is in theradio frequency range.
 59. A system as defined by claim 57 wherein thefrequency of said excitation is in the range between about 1 MHz and 40MHz.
 60. A system as defined by claim 59 wherein said conductive meanscomprise opposing spaced rows of conductors disposed in opposing spacedrows of boreholes in said bed.
 61. A system as defined by claim 60wherein said rows of conductors comprise three spaced rows ofconductors.
 62. A system as defined by claim 61 wherein the conductorsof the central row are of substantially shorter length than theconductors of the outer rows so as to reduce radiation at the ends ofsaid conductors.
 63. A system as defined by claim 62 wherein thefrequency of said excitation is selected such that a half wavelength ofelectromagnetic energy in the region beyond the center conductor issubstantially greater than the spacing between the outer rows to giverise to a cutoff condition in said region.
 64. A system as defined byclaim 59 wherein the frequency of said excitation is selected as afunction of the electrical lossiness of the formations in said volume tobe sufficiently low such that the l/e attenuation distance of theelectric field in any direction in said volume is more than twice thephysical dimension of said volume in that direction.
 65. A system asdefined by claim 57 wherein said conductive means comprise opposingspaced rows of conductors disposed in opposing spaced rows of boreholesin said bed.
 66. A system as defined by claim 57 wherein the frequencyof said excitation is selected as a function of the electrical lossinessof the formations in said volume to be sufficiently low such that thel/e attenuation distance of the electric field in any direction in saidvolume is more than twice the physical dimension of said volume in thatdirection.
 67. A system for in situ heat processing of a tar sanddeposit, comprising:a plurality of conductive means inserted in saiddeposit and bounding a particular volume of said deposit; electricalexcitation means for establishing alternating electric fields in saidvolume; the frequency of said excitation means being selected as afunction of the volume dimensions so as to establish substantiallynon-radiating electric fields which are substantially confined in saidvolume; whereby volumetric dielectric heating of the deposit will occurto effect approximately uniform heating of said volume.
 68. A system asdefined by claim 67 wherein the frequency of said excitation is in theradio frequency range.
 69. A system as defined by claim 68 wherein thefrequency of said excitation is selected as a function of the electricallossiness of the formations in said volume to be sufficiently low suchthat the skin depth of the electric field in any direction in saidvolume is more than twice the physical dimension of said volume in thatdirection.
 70. A system as defined by claim 67 wherein the frequency ofsaid excitation is selected as a function of the electrical lossiness ofthe formations in said volume to be sufficiently low such that the skindepth of the electric field in any direction in said volume is more thantwice the physical dimension of said volume in that direction. .Iadd.71. A system for in situ heat processing of hydrocarbonaceous earthformations, comprising:a waveguide structure comprising a plurality ofelongate electrodes and configured such that the direction ofpropagation of aggregate modes of wave propagation therein isapproximately parallel to an elongate axis of said electrodes andbounding a particular volume of earth formations as a dielectric mediumbounded therein; and means for supplying electromagnetic energy to saidwaveguide structure at a frequency selected to confine saidelectromagnetic energy in said structure and to dissipate saidelectromagnetic energy to the earth formations, thereby to substantiallyuniformly heat the bounded volume. .Iaddend..Iadd.
 72. A system for insitu heat processing of hydrocarbonaceous earth materials, comprising: awaveguide structure having an elongate shape which penetrates and boundsa particular volume of earth formations therein and wherein theaggregate direction of propagation of electromagnetic wave modes in saidstructure is in a direction approximately parallel to an elongate axisof said structure; and means for supplying electromagnetic energy tosaid waveguide structure at a frequency selected to confine said energyand to dissipate said electromagnetic energy to said bounded volumethereby to substantially uniformly heat said bounded volume. .Iaddend..Iadd.
 73. A system for in situ heat processing of hydrocarbonaceousearth formations, comprising: field confining means bounding aparticular volume of earth formations and forming an elongate waveguidestructure having a direction of aggregate electromagnetic wavepropagation mode direction in a direction approximately parallel to anelongate axis of said structure; and means for supplying electromagneticenergy to said waveguide structure at a frequency to confine saidelectromagnetic energy in said structure and to cause dielectric heatingof said bounded volume to a substantially uniform degree..Iaddend..Iadd.
 74. A system for in situ heat processing ofhydrocarbonaceous earth formations comprising: a plurality of electrodesplaced in a pattern bounding a particular volume of hydrocarbonaceousearth formation, said pattern defining a waveguide structure having saidvolume bounded therein as a dielectric medium; and, means for applyingan alternating current to said electrodes in the frequency range of theorder of 100 kilohertz to 100 megahertz, the frequency of said currentbeing selected as a function of a volume dimension so as to establishsubstantially non-radiating and uniform electromagnetic fields in saidvolume, thereby obtaining volumetric dielectric heating of said volumeto a temperature sufficient to permit production of hydrocarbonaceouscomponents thereof. .Iaddend. .Iadd.
 75. A system of in situ heatprocessing of hydrocarbonaceous earth formations comprising: a patternof conductors bounding a particular volume of hydrocarbonaceous earthformation, said pattern defining an unbalanced transmission linestructure having said bounded volume integral therewith as a dielectricmedium; and means for supplying alternating current in the frequencyrange of the order of 100 kilohertz to 100 megahertz, to saidconductors, the frequency of said current being selected as a functionof at least one volume dimension so as to establish substantiallynon-radiating electromagnetic fields in said volume. .Iaddend..Iadd. 76.A system for in situ heat processing of hydrocarbonaceous earthformations comprising: a substantially tri-plate pattern of electrodesplaced in a particular volume of hydrocarbonaceous earth formation andforming a waveguide structure having said volume bounded therein as adielectric medium wherein adjacent portions of electrodes within a plateare at approximately the same potential; and means for supplying a timevarying electric field in the frequency range from 100 kilohertz to 100megahertz to said electrodes so as to establish substantiallynon-radiating electric fields in said volume. .Iadd.
 77. A system for insitu heat processing of hydrocarbonaceous earth formations comprising:waveguide means formed by a pattern of electrodes placed in a particularvolume of hydrocarbonaceous earth formation to bound said volume thereinas a dielectric medium; and means for supplying alternating current tosaid waveguide structure at a frequency of the order of from 100kilohertz to 100 megahertz to effectively confine electromagnetic fieldsin said structure and to effect substantially uniform dielectric heatingof said volume. .Iaddend..Iadd.
 78. A system for in situ heat processingof hydrocarbonaceous earth formations comprising: an unbalancedtransmission line structure deployed in a particular volume ofhydrocarbonaceous earth formation, said structure bounding said volumeand employing said formation material as a dielectric medium therein;and means for supplying electrical energy having a frequency of theorder of from 100 kilohertz to 100 megahertz to said transmission linestructure, thereby confining said energy in said structure and providingdielectric heating to a controllable degree in said volume. .Iaddend..Iadd.
 79. A system for in situ heat processing of hydrocarbonaceousearth formations comprising: a waveguide structure formed by bounding aparticular volume of earth formations with a pattern of electrodesbounding said volume and including said volume as a dielectric mediumtherein; and means for establishing alternating electromagnetic fieldsin the frequency range between 100 kilohertz and 100 megahertz in saidbounded volume, the frequency of said alternating fields being selectedas a function of a volume dimension, thereby causing volumetricdielectric heating of said volume to an approximately uniform degree..Iaddend..Iadd.
 80. A system for in situ heat processing ofhydrocarbonaceous earth formations comprising: electrode means boundinga particular volume of earth formations in such a manner as to comprisea waveguide structure having said volume bounded therein as a dielectricmedium; and means for supplying electromagnetic energy to said waveguidestructure at a frequency selected to confine said energy substantiallyin said volume and to cause heating of said volume by displacementcurrents to a substantially uniform degree in said volume. .Iaddend..Iadd.
 81. A system for in situ heat processing of hydrocarbonaceousearth formations, comprising: electrode means bounding a particularvolume of earth formations in such a manner as to comprise an unbalancedtransmission line structure having said volume bounded therein as adielectric medium; and means for supplying electromagnetic energy tosaid unbalanced transmission line at a frequency selected to causeheating of said volume by displacement currents to a substantiallyuniform degree in said volume. .Iaddend..Iadd.
 82. A system for in situheat processing of hydrocarbonaceous earth formations comprising:electrode means bounding a particular volume of earth formations in sucha manner as to comprise an approximately tri-plate transmission linestructure having said volume bounded therein as a dielectric medium; andmeans for supplying electromagnetic energy to said triplate transmissionline structure at a frequency and field intensity selected to causeheating of said volume to a substantially uniform degree in said volumewithout significant heat loss to the adjacent unbounded regions andwithout electrical breakdown of said bounded volume. .Iaddend. .Iadd.83. A system for in situ heat processing of hydrocarbonaceous earthformations, comprising a waveguide structure comprising a plurality ofelectrodes bounding a particular volume of earth formations as adielectric medium bounded therein; and means for supplyingelectromagnetic energy to said waveguide structure at a frequencyselected to dissipate said electromagnetic energy substantially only tosaid bounded medium thereby to substantially uniformly heat said boundedvolume. .Iaddend..Iadd.
 84. A system for in situ heat processing ofhydrocarbonaceous earth formations, comprising: an unbalancedtransmission line structure comprising a plurality of electrodesbounding a particular volume of earth formations as a dielectric mediumbounded therein; and means for supplying electromagnetic energy to saidunbalanced transmission line structure at a frequency selected tosubstantially confine said energy to said structure and to dissipatesaid electromagnetic energy to said dielectric medium by displacementcurrent heating thereof, thereby to substantially uniformly heat saidbounded volume without significant heat loss to the adjacent unboundedregions and without electrical breakdown of said bounded volume..Iaddend. .Iadd.
 85. A system for in situ heat processing ofhydrocarbonaceous earth formations, comprising: an approximatelytri-plate transmission line structure comprising a plurality ofelectrodes bounding a particular volume of earth formations as adielectric medium bounded therein; and means for supplyingelectromagnetic energy to said approximately tri-plate transmission linestructure at a frequency and field intensity selected to dissipate saidelectromagnetic energy to said dielectric medium, thereby tosubstantially uniformly heat said bounded volume without significantheat loss to the adjacent unbounded regions and without electricalbreakdown of said bounded volume. .Iaddend..Iadd.
 86. A system for insitu heating a volume of hydrocarbonaceous earth formation to anelevated temperature comprising: electrical excitation means forproviding an electrical waveform of a frequency of the order of from 100kilohertz to 100 megahertz; a conductor array located approximatelycentrally in said volume to which the electrical waveform is applied,said central conductor array comprising a line of conductors inserted inboreholes in the formation, wherein adjacent conductors in the line areseparated by a distance of about 1/8 of a wavelength of less of theelectrical waveform; and a bounding conductor array comprising at leastone line of electrical conductors inserted in boreholes in theformation, adjacent of said conductors in a line being separated byabout 1/8 of a wavelength or less of the electrical waveform whereinbounding conductors are at approximately the same potential as theadjacent unbounded earth formations whereby radiation of electricalenergy outside the volume of the hydrocarbonaceous earth formation isminimized. .Iaddend..Iadd.
 87. A method of heating a volume ofhydrocarbonaceous earth formations to an elevated temperaturecomprising: applying an electrical waveform to a first row of elongatedconductors penetrating a volume of the formation, adjacent conductorsbeing separated by a distance less than 1/8 of the wavelength of theelectrical waveform; confining the electromagnetic field in the volumeby bounding said volume with at least two rows of elongated conductors,adjacent conductors in a row being separated by a distance less than 1/8of the wavelength of the electrical waveform; and varying at least oneof; (a) the frequency of the electrical waveform; (b) the physicallength of individual conductors in a row of conductors, (c) the seriescapacitance of conductors in a row; (d) the effective electrical lengthof conductors in a row; to facilitate uniform heating of the formationin the direction of the principal axis of the elongated conductors..Iaddend..Iadd.
 88. A method for in situ heat processing ofhydrocarbonaceous earth formations comprising the steps of: placing aplurality of electrodes into a particular volume of hydrocarbonaceousmaterial in a pattern which bounds said volume and defines an unbalancedtransmission line structure having said bounded volume present as adielectric medium bounded therein; applying alternating current at aradio frequency on the order of from 100 kilohertz to 100 megahertz tosaid electrodes, said radio frequency being chosen as a function of avolume dimension so as to establish substantially non-radiatingelectromagnetic fields which are substantially confined in said volume,thereby effecting approximately uniform heating of said volume to atemperature sufficient to permit production of hydrocarbonaceouscomponents thereof. .Iaddend..Iadd.
 89. A method for in situ heatingprocessing of hydrocarbonaceous earth formations comprising the stepsof:placing a plurality of electrodes into a particular volume ofhydrocarbonaceous material in a pattern which bounds said volume anddefines a waveguide structure having said bounded volume present as adielectric medium bounded therein; applying alternating current at aradio frequency on the order of 100 kilohertz to 100 megahertz to saidelectrodes, said radio frequency being chosen as a function of at leastone volume dimension so as to establish substantially nonradiatingelectromagnetic fields which are substantially confined in said volume,thereby effecting approximately uniform heating; modifying theelectromagnetic field pattern so as to time average the electromagneticfield in said volume to enhance the uniformity of heating of saidvolume. .Iaddend. .Iadd.
 90. A method for in situ heat processing ofhydrocarbonaceous earth formations, comprising the steps of: forming aplurality of holes which bound a particular volume of hydrocarbonaceousmaterial and spaced from each other so as to define an approximatelytriplate structure having said bounded volume of hydrocarbonaceousmaterial present as a dielectric medium bounded therein; insertingelectrical conductors into said holes; and, applying alternating currentat a radio frequency on the order of from 100 kilohertz to 100 megahertzto said conductors, said radio frequency being chosen as a function ofat least one volume dimension so as to establish substantiallynonradiating electromagnetic fields which are substantially confined insaid volume, thereby effecting approximately uniform heating of saidvolume. .Iaddend..Iadd.
 91. A method for in situ heat processing ofhydrocarbonaceous earth formations, comprising the steps of: enclosing aparticular volume of earth formations on at least two sides thereof witha plurality of spaced electrodes to define a waveguide structure havingsaid enclosed volume present therein as a dielectric medium; andestablishing alternating electromagnetic fields in the frequency rangebetween 100 kilohertz to 100 megahertz in said enclosed volume, thefrequency of said alternating fields being selected as a function of avolume dimension, so as to establish substantially non-radiating,confined, electromagnetic fields in said volume, thereby causingvolumetric dielectric heating of said volume to effect approximatelyuniform heating of said volume. .Iaddend..Iadd.
 92. A method for in situheat processing of hydrocarbonaceous earth formations comprising thesteps of:bounding a particular volume of earth formations with awaveguide structure comprising elongate electrodes having outerelectrodes which are at approximately the same potential as the adjacentunbounded earth formations; and propagating electromagnetic energythrough the waveguide structure in an aggregate mode of propagationgenerally parallel to the direction of an elongate axis of saidelectrodes, thereby substantially confining the electromagnetic energyin the waveguide structure and uniformly heating the bounded volume ofearth formations. .Iaddend. .Iadd.
 93. A method for in situ heatprocessing of hydrocarbonaceous earth formations, comprising the stepsof: bounding a particular volume of earth formations with a transmissionline structure having an inner elongate shaped propagating electrodestructure and an outer elongate shaped electrode structure which is atapproximately the same potential as the adjacent unbounded earthformations; and propagating modes of electromagnetic energy in saidstructure in an aggregate direction generally parallel to an elongateaxis of said propagating electrodes, thereby confining saidelectromagnetic energy in said bounded volume and uniformly heating saidbounded volume. .Iaddend..Iadd.
 94. A system for in situ heat processingof hydrocarbonaceous earth formations, comprising: a multi mode cavitystructure comprising a plurality of elongate electrodes and configuredsuch that the direction of wave propagation of a particular mode isparallel to an elongate axis of at least one set of said electrodes,said multi mode cavity structure bounding a particular volume of earthformations as a dielectric medium bounded therein wherein the outermostelectrodes are at approximately the same potential as the adjacentunbounded earth formations; and means for supplying electromagneticenergy to said multi mode cavity structure at a frequency selected toconfine said electromagnetic energy in said structure and to dissipatesaid electromagnetic energy to the earth formations; thereby tosubstantially uniformly heat the bounded volume. .Iaddend..Iadd.
 95. Thesystem of claim 94 and further including means for time averaging saidelectromagnetic energy along the direction of propagation, thereby toenhance the uniformity of heating of the bounded volume of earthformations. .Iaddend..Iadd.
 96. A system for in situ heat processing ofhydrocarbonaceous earth formations, comprising: a waveguide structurehaving a plurality of rows of conductors, the spacing of conductors in arow being less than the spacing of said rows of conductors and boundinga particular volume of earth formations as a dielectric medium boundedtherein; and, means for supplying electromagnetic energy to saidwaveguide structure at a frequency selected to dissipate saidelectromagnetic energy substantially only to said bounded medium,thereby to substantially uniformly heat said bounded volume..Iaddend..Iadd.
 97. The system of claim 96 and further including meansfor time averaging said electromagnetic energy along a direction of itspropagation in said waveguide structure. .Iaddend.